The present invention relates generally to the field of voltage controlled oscillators and more particularly to distributed voltage controlled oscillators (DVCO""s) that are tunable over a wide band of microwave frequencies.
Wireless broadband technology offers the prospect of mobile alternatives to high speed, wired, voice and data communication systems (e.g., fiber optic or copper wire broadband transmission systems). As with conventional radio frequency (RF) devices, an important component for up-conversion (transmission) or down-conversion (reception) in microwave broadband communication devices is the local oscillator, and particularly, the voltage-controlled oscillator (VCO), that operates in the frequency range of the carrier signal. Thus, the VCO is an essential component for up/down conversion of the transmitted signal. Typical design criteria for VCO""s are frequency stability, high output level, tunablity, low phase noise, small packaging and low cost. Further, in order to increase the bandwidth of the transmitted RF signal, thereby increasing the rate of data transmission at which such wireless broadband communications systems can operate, VCO""s in transmitters must be capable of generating very high, microwave carrier frequencies, that is, in the 10 gigahertz range and above. One example is the 28 GHz band of local multipoint distribution services (xe2x80x9cLMDSxe2x80x9d) systems.
Moreover, with the increasing market demand for more powerful and smaller wireless communications systems with greater bandwidth capacity, such as wireless networked portable computers, personal digital assistants (xe2x80x9cPDAxe2x80x9d) and other specialty communications devices, and the convergence of voice and data, there is a need for high frequency, broadband tunable VCO""s that can be integrated into the microwave front-end circuits (transceivers) that are themselves integrated with digital back end circuits on a single integrated circuit (xe2x80x9cICxe2x80x9d) chip.
Unfortunately, existing lumped solutions for such integrated, high frequency oscillators are inadequate. For example, while it is possible to design a tunable LC resonant tank oscillator circuit on a silicon substrate at up to 10 GHz, it becomes excessively difficult to achieve a wide tuning range and good phase noise as the frequency of operation approaches the fmax, or cut off frequency, of the transistors. This is mainly due to the trade off between the self-resonance frequency and the quality factor, Q, of the integrated inductors and varactors, which is very low for operation at frequencies above the C-band (i.e. above about 6.5 GHz). This trade off becomes prohibitive as the operating frequency increases.
Thus, there exists a need for a microwave voltage-controlled oscillator (xe2x80x9cVCOxe2x80x9d) that (1) is small, i.e. capable of being designed as part of an integrated circuit (IC) package; (2) is low cost; (3) provides stable operation; and (4) is capable of wide band tuning.
The distributed oscillator has recently been considered as a possible low-cost microwave VCO solution in CMOS radio frequency integrated circuits (xe2x80x9cRFIC""sxe2x80x9d), due to its ability to operate at frequencies close to the intrinsic cutoff frequencies of the transistors. The distributed oscillator originates from the distributed amplifier, which has been studied for many years. For example, Skvor, et al. proposed to build a VCO by operating a distributed amplifier in the reverse gain mode, using the output from the idle drain load as the feedback output. See, xe2x80x9cNovel Decade Electronically Tunable Microwave Oscillator based on the Distributed Amplifier,xe2x80x9d Electronics Letters, vol. 28, no. 17, pp. 1647-1648, August 1992. Further, a 4 GHz, distributed oscillator was assertedly demonstrated using discrete pHEMTs and microstrip lines on a printed circuit board (PCB). Divina L., Skvor Z., xe2x80x9cThe Distributed Oscillator at 4 GHz,xe2x80x9d IEEE Trans. MTT, vol. 46, no. 12, pp. 2240-2243, December 1998. Another group recently assertedly showed an integrated (with off-chip termination and bias) distributed oscillator operating at 17 GHz without any tuning capability using 0.18 mm CMOS technology. The forward gain mode instead of reverse gain mode was used and assertedly demonstrated that CMOS is viable for oscillator applications at microwave frequencies. See Kleveland B., et al., xe2x80x9cMonolithic CMOS Distributed Amplifier and Oscillator,xe2x80x9d IEEE Int. Solid-State Circ. Conf., Paper MP 4.3, February 1999.
Despite these apparent advances, tuning remains a problem since distributed VCO""s (xe2x80x9cDVCO""sxe2x80x9d) are used at frequencies close to the device fT, where there is not enough gain to lose in tuning circuitry. Consequently, the addition of extra integrated varactors with low Q is not a favorable option due to their high loss which further deteriorates with frequency. Nor can the reverse mode tuning scheme described in the above-referenced Skvor et al. article be used due to the limited transistor gain in CMOS technologies. Therefore, a new tuning scheme must be devised.
Accordingly, it should be appreciated that there exists a definite need for a sufficiently tunable, operatively stable, and relatively low cost and integrated DVCO.
The present invention, which addresses these needs, sufficiently resides in a tunable distributed voltage control oscillator which operates at very high frequencies, is advantageously tunable across a relative very wide frequency range and is integrable on an integrated chip.
In accordance with the present invention, integrated, tunable distributed voltage-controlled oscillators (DVCO""s) and methods for tuning such oscillators over a wide microwave frequency range are disclosed. The DVCO""s include two substantially parallel transmission lines, at least one three terminal active device disposed between the lines, and a tuning circuit connected to the active device that tunably controls the frequency on the lines. More particularly, the DVCO""s include (1) an input transmission line with a loaded characteristic impedance having an input at one end, and an output at an opposite end that is terminated by a wave-absorbing termination that matches the loaded characteristic impedance of the input line and biased with a dc voltage; (2) an output transmission line with a loaded characteristic impedance, having an input at one end that is terminated by a wave-absorbing termination that matches the impedance of the output transmission line and biased with a dc biasing voltage, and an output at a second end that is connected to the input of the input transmission line; (3) at least one three-terminal active device with a transconductance, gm, having a biasing input terminal connected to the input transmission line and an output terminal connected to the output transmission line; and (4) a tuning circuit connected to the active device that controls the time delay of the signal propagating on at least one of the transmission lines which, in turn, controls the oscillation frequency of the signal transmitting on the transmission lines. The output line preferably runs substantially in parallel with the input line.
In one detailed aspect of the invention, the tuning circuit is a current-steering circuit that operates in conjunction with the active device to controllably adjust the effective electrical length of the output transmission line. Electrically reducing the electrical length of the transmission line increases the frequency on the line and increasing the length decreases its frequency.
In another more detailed aspect of the present invention, the tuning circuit comprises an ac coupling capacitor disposed between the output of the output transmission line and input of the input transmission line. This capacitor enables the independent control of voltage on each transmission line, such that by adjusting the dc bias voltage of the input transmission line, the nonlinear capacitances and transconductances, of the at least one active device is controllably adjusted. In this way, the time delay and thus oscillation frequency of the transmission lines is adjusted. The capacitive coupling technique enables larger range but more coarse frequency tuning relative to the current steering technique described above.
The at least one active device is typically an amplifying microwave transistor, such as a CMOS or bipolar transistor. However, it can be any three terminal gain device such as an HBT vacuum tube.
In another aspect of the invention, the tunable DVCO includes an input transmission line as described above, an output transmission line as described above and an output line tuning section (xe2x80x9cOLTxe2x80x9d) having an input connected to the input transmission line and two outputs connected to the output transmission line and separated by a transmission line segment of a given length on the output line. The OLT both amplifies the signal propagating on the output transmission line and controllably alters the electrical length of the output transmission line to adjust the frequency on the line.
The OLT includes a two three-terminal active device (such as gain transistors) and a dc current source that adjust the current between the two active devices. More particularly, the first three-terminal active device includes a control input terminal connected to the input line, an output terminal tapped to the output line, and a current input terminal. The second three-terminal active device includes a biasing input terminal connected to the input line at a tap point in common with the input terminal of the first transistor, an output terminal tapped to the output line at a point downstream from the tapping point of the output terminal of the first transistor in the direction of the output of the OTL, and a current input terminal. Importantly, the output terminals of the two transistors are spaced apart by an output transmission line segment of a given length. The dc current source controllably inversely distributes its current between the current input terminals of the two active devices, such that as the current to the two active devices is adjusted, the effective length of the output transmission line is varied, thereby controllably tuning the oscillation frequency.
In a preferred embodiment, the DVCO described in the preceding two paragraphs also includes an input line tuning section (ILT) that is completely complementary to the OLT and is connected to the input transmission line and output transmission line in parallel with the OLT. The purpose of the ILT is to balance the phase delay mismatch between the input and output lines introduced by the OLT. In particular, the complementary ILT includes a first complementary active device, a second complementary active device and a dc current source. The first complementary active device (such as a transistor) includes a control input terminal connected to the input line, an output terminal tapped to the output line, and a current input terminal. The second complementary active device has an output terminal connected to the output line at a tap point in common with the output terminal of the first complementary active device, an input terminal tapped to the input line at a point downstream from the tapping point of the input terminal of the first active device in the direction of the output of the input line and a current input terminal. The two input terminals are spaced apart by an input transmission line segment of a given length. The dc current source controllably inversely distributes its current supply between the current input terminals of the two complementary active devices, such that as the current to the two active devices is adjusted, the effective length of the input transmission line is varied, thereby controllably balancing the delay mismatch created by the OLT on the output transmission line.
As an alternative to the embodiment that describes the oscillator designed with an OLT (without a complementary delay balancing section), the oscillator may instead be designed with an ILT.
In yet a more particular aspect of the invention, the dc current source of the OLT includes a pair of active devices, each having a voltage input. These inputs together define a differential control voltage. Further, the dc current source of the ILT includes a second pair of complementary active devices each having a voltage these inputs also define a differential control voltage.
In the preferred embodiment, the DVCO includes at least one more OLT and complementary ILT connected to the input and output transmission lines. Each additional pair of OLT and ILT that is placed in the line increases the gain of the oscillator circuit.
Other features and advantages of the present invention should become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.